Broadband Wavelength-swept Raman Laser for Fourier-domain Mode Locked Swept-source OCT

Transcription

1 Journal of the Otical Society of Korea Vol. 13, No. 3, Setember 2009, DOI: /JOSK Broadband Wavelength-wet Raman Laer for Fourier-domain Mode Locked Swet-ource OCT Hyung-Seok Lee, Eun-Joo Jung, Myung-Yung Jeong, and Chang-Seok Kim* Deartment of Cogno Mechatronic Engineering, Puan National Univerity, Puan , Korea (Received June 8, 2009 : revied Augut 10, 2009 : acceted Setember 1, 2009) A novel broadband wavelength-wet Raman laer wa ued to imlement Fourier-domain mode locked (FDML) wet-ource otical coherence tomograhy (SS-OCT). Intead of a conventional emiconductor otical amlifier, thi tudy ued broadband otical fiber Raman amlification, over 50 nm centered around 1545 nm, uing a multi-wavelength otical uming cheme, which wa imlemented with the four laer diode at the center wavelength of 1425, 1435, 1455 and 1465 nm, reectively, and the maximum oerating ower of 150 mw each. The oerating wet frequency of the laer wa determined to 16.7 khz from the FDML condition of 12 km otical fiber in the ring cavity. The OCT image were obtained uing the novel broadband wavelengthwet Raman laer ource. Keyword : Otical coherence tomograhy, Fiber Raman amlifier, Wavelength-wet fiber laer OCIS code : ( ) Otical coherence tomograhy; ( ) Laer, Raman; ( ) Laer fiber I. INTRODUCTION Otical coherence tomograhy (OCT) emloy low coherence interferometry to acquire micron-cale croectional image non-invaively [1-3]. Fourier-domain OCT ha many advantage over conventional timedomain OCT, uch a a higher acquiition eed without mechanical delay line and a higher ignal-to-noie ratio. A romiing method for high-eed imaging i offered by wet-ource (SS) OCT [1-5]. The erformance of high-eed and high-reolution SS-OCT imaging deend on the effective wee rate and tuning of the ectral range of the wavelength-wet laer ource, reectively. Generally, mot wavelength-wet laer for SS- OCT emloy a emiconductor otical amlifier (SOA) a the gain medium [1-4]. The tyical carrier relaxation time of a SOA i everal nanoecond and it gain band normally rovide a bandwidth le than 100 nm becaue the wee rate and ectral range are limited by the carrier relaxation time and entire gain band of the gain medium, reectively. Fourier domain modelocked (FDML) technique have been reorted to increae the weeing eed of wavelength-wet laer uing *Correonding author: ckim@uan.ac.kr additional delayed length of otical fiber in the laer cavity [2]. However, the long length of otical fiber over ~ km induce additional inertion lo of the cavity, which i unhelful for laing oeration. Recently, we rooed that an additional delayed length of an otical fiber can be ued a a gain medium intead of a lo medium when fiber Raman amlification i generated by a high ower um light [6]. In thi tudy, a novel broadband wavelength-wet Raman laer wa demontrated uing a multi-wavelength otical uming cheme to imlement FDML SS-OCT imaging. Thi i the firt exerimental demontration of a broadband SS-OCT baed on a fiber Raman amlifier a a broad gain medium over 50 nm, comared with the reviou reort of narrower Raman gain uing a ingle-wavelength otical uming cheme [6,7]. A high ower otical um ource have become commercially available, a fiber Raman amlification baed on timulated Raman cattering in ilica otical fiber ha been tudied widely, articularly in wavelength-diviion-multilexed (WDM) telecommunication ytem alication [8-13]. The fiber Raman amlifier ha everal advantage, uch a low noie, arbitrary gain band determined on the band of the otical um ource, high temerature tability and intantaneou carrier relaxation time of

2 Broadband Wavelength-wet Raman Laer - Hyung-Seok Lee et al. 317 the order of everal femto-econd for ilica [9]. It i exected that the alication of a fiber Raman amlifier canbeextendedtobiomedicalimagingbaedonthee exerimental reult due to the increaing demand for higher-eed and a wider-gain band for the real time and high reolution OCT imaging, reectively. II. WAVELENGTH SWEPT RAMAN LASER Figure 1 how a chematic diagram of the wavelengthwet laer baed on fiber Raman amlifier gain. The laer conited of a 12 km Raman gain otical fiber, a wavelength diviion multilexing (WDM) couler to receive a multi-wavelength otical um ower, two iolator for a unidirectional configuration, a olarization controller (PC), fiber Fabry-Perot tunable filter (FFP-TF) and an outut couler into a SS-OCT ytem. For an efficient and table Raman gain roce, a tandard 11 km dierion hifted fiber (DSF) and a 1 km dierion comenation fiber (DCF) are otimally ued for the Raman gain otical fiber medium becaue a normal dierion cheme i referred in the gain wavelength region of aroximately 1500 ~ 1600 nm. When the zero dierion wavelength,,of11kmdsf i aroximately 1550 nm, the total dierion value of the 12 km gain media will take the normal dierion characteritic over the 1550 nm region with the aid of a 1 km DCF [8]. A 14XX/15XX WDM couler wa ued in the ring laer cavity for the effective couling of the multi-wavelength uming light below 1500 nm and the round-tri ignal light over 1500 nm. A broadband Raman gain region i needed for higher reolution OCT imaging. In thi tudy, a um combining couler wa ued to generate a multi-wavelength uming cheme with four laer diode. The center wavelength of each LD wa 1425, 1435, 1455 and 1465 nm, reectively, and the maximum oerating ower wa 150 mw for each LD. Fig. 2 how a ectral comarion of the Raman amlifier; in each cae when LD 1 wa turned on, LD 1 and 2 were turned on, and LD 1, 2, 3, and 4 were turned on, reectively. The reult how that the wider uming wavelength induce a broader Raman gain wavelength region. Conidering the interaction between the um and Stoke wave to find the Raman threhold, the evolution of the ignal and um ower level can be decribed uing the following couled equation [11], di = g RI I α I (1) dz di ω = g RI I α I (2) dz ω where I i the Stoke intenity, I i the um intenity, i the Raman-gain coefficient and the abortion coefficient and denote the fiber lo at the Stoke and um frequencie. The firt term on the right ide of each equation decribe the nonlinear timulated roce and coule the two wave. The econd term on the right ide are due to the linear abortion in the fiber medium. From a numerical imulation of the cacaded Raman frequency hift [10,11], it wa confirmed that a wider ditribution of multi-wavelength uming below 1500 nm will reduce the broader ditribution of Raman gain above the 1500 nm region. A novel broadband wavelength-wet FDML Raman laer wa demontrated baed on the broadband Raman gain amlification. Conidering the length of a 12 km Raman gain fiber, the oerating frequency of a fiber Fabry-Perot tunable filter wa determined to be 16.7 khz under the FDML condition [2]. Fig. 3 how the outut ectrum of a Raman laer at nm Raman Gain Otical Fiber Intenity [dbm] -70 LD 1,2,3,4 On LD 1,2 On LD 1 On -80 FIG. 1. Setu of wavelength-wet laer including the Raman amlifier baed on multi-wavelength uming cheme (LD: Laer Diode, FFR-TF: Fiber Fabry-Perot Tunable Filter) FIG. 2. Comarion of variou ectra of Raman amlifier when LD 1 i on, LD 1 and 2 are on, and LD 1,2,3, and 4 are on, reectively.

3 318 Journal of the Otical Society of Korea, Vol. 13, No. 3, Setember 2009 Intenity [dbm] FIG. 4. Scheme of exerimental etu for OCT imaging (ND: Neutral Denity Filter, GM: Galvano Mirror) FIG. 3. Outut ectrum of Raman laer when FFP-TF i not weeing, Outut ectrum in eak hold mode of the wavelength wet Raman laer when FFP-TF i weeing reeatedly. when the FFP-TF wa not weeing. The reult how that the line-width i 0.07 nm and the ignal-to-noie of the laer i larger than 60 db. Fig. 3 how the meaurement reult of the overall outut ectrum of the wavelength wet Raman laer when the FFP-TF i weeing at a 16.7 khz inuoidal frequency. At the eak hold mode of the otical ectrum analyzer (OSA), the 3 db bandwidth of 50 nm wa centered at aroximately 1543 nm and the extinction ratio wa aroximately 50 db. Comared to the ingle wavelength uming for the 30 nm bandwidth [7], the bandwidth of the weeing range wa almot doubled by the uggeted multi-wavelength uming cheme. III. OCT IMAGING Figure 4 how the exerimental etu for OCT imaging baed on the wavelength wet Raman laer. Two tunable directional couler and two circulator were ued to contruct a Mach-Zehnder interferometer. Uing the tunable directional couler, the otical ower between the amle ort and reflecting mirror ort were controlled manually. The amle ath wa canned laterally uing the galvano mirror and objective len. A cro ectional image wa roduced by tranverely canning the beam acro the amle while collecting a reflecting rofile at each oint. The converted reflecting intenitie were recorded on a gray cale image a a function of the tranvere and axial ditance. The deth oint of 1024 were acquired for the erie of image and each image wa comoed of 512 axial 256 tranvere ixel.fig.5howthetemoraltranientintenity rofile of the wavelength-wet laer outut, which wa meaured uing an ocillocoe. The electrical intenity rofile of the laer outut wa reented a the ine modulation waveform wa alied to the canning filter. Comared to the imilar trace with a SOA gain medium, the outut howed relatively higher fluctuation over the original ignal rofile [7] but the interferogram information can till be induced to convert an OCT image from it. Fig. 5 how a table interferogram fringe ignal between the reference mirror and amle arm with a mirror from the Mach-Zehnder tye OCT. Figure 6 how a deth encoded ignal uing the dicrete Fourier tranform (DFT) for the amle of the four cover glae. The two dimenional OCT amle image i alo demontrated in Fig. 6. All four cover glae and a few air ga between the cover glae can be clearly ditinguihed within the tomograhy. Fig. 7 how an OCT image of a human tooth. Comared to the conventional light ource for an OCT around the 1300 nm, 1000 or 800 nm region, thi novel wavelength wet Raman laer around the 1550 nm region i believed to be ueful for ecific biological tiue, uch a the human tooth, where the dominant lo factor i cattering-induced light attenuation rather than water abortion [14]. The variability in enamel morhology can clearly delineate the junction between the enamel and dentin layer. The inet how an arrow indicating the can range of the tranvere direction for OCT image.

4 Broadband Wavelength-wet Raman Laer - Hyung-Seok Lee et al. 319 FIG. 5. Temoral tranient intenity rofile of wavelength-wet outut meaured by ocillocoe, Interferogram fringe ignal from Mach-Zehnder tye OCT with mirror amle. FIG. 6. Deth encoded ignal uing Dicrete Fourier Tranform 2-dimenional OCT image of four lide glae. max min log 500 μm FIG.7. OCT image of a human tooth amle. Inet icture how the urface location of the tranvere canning of imaging. a novel broadband FDML wavelength wet Raman laer. The FDML Raman laer cheme emloy the delayed length of an otical fiber a a gain medium, intead of the lo medium in a conventional FDML oeration. Sectral broadening of Raman gain wa demontrated exerimentally uing multi-wavelength uming laer diode. ACKNOWLEDGMENT Thi work wa uorted by the IT R&D rogram of MKE/IITA [2008-F ], Korea. IV. CONCLUSION Thi tudy demontrated variou OCT image uing REFERENCES 1. S. H. Yun, G. J. Tearney, J. F. de Boer, N. Iftimia, and

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